Python's a lingua franca in AI/NN because it was already a dominant language in scientific computing. Its dominance in scientific computing grew steadily through the 1990s and 2000s, for a few reasons:

1) Python -- specifically CPython -- made it easy to wrap existing, thoroughly tested high performance libraries in Python APIs. So, you got easy access to things like GSL and BLAS and LAPACK, but you get to call numpy.linalg.svd instead of GESDD.

2) Python was a general purpose language, unlike R or MATLAB, so you could extend existing systems to do more without running into a wall.

3) Python was a heck of a lot less effort to use than C++.